The Gloss of Injection-Moulded Recovered Polypropylene

Abstract In our recent work, it was shown that the rate of filling during injection moulding did not significantly affect the tensile and impact properties of recovered and virgin polypropylene, whereas significant changes in the properties were observed after ageing at 100 °C. The purpose of the present work was to extend the former study to include the effects of processing and ageing on surface-appearance-related properties, in particular the gloss. In-mould rheology and gate-seal analyses were performed in order to select the filling and post-filling processing windows, respectively. The influences of filling rate, holding and cooling times on the gloss measured in the gate area and at the far end of the moulded specimens were evaluated. The mould used had an embossed surface giving test plaques with a matte appearance. The results showed that the gloss varied by about 30 to 40% within the filling window obtained by in-mould rheology. The slight gloss difference between the two PP-grades noted at the lowest injection speed vanished with increasing filling rate. Gloss differences were observed between the gate area and the far end of each processed moulding, again at the lowest injection speed. The materials processed at a high filling rate exhibited a fairly constant gloss as the holding time increased regardless of the position of the analysed region, whereas a more significant decrease was noted in the case of the mouldings processed at the low filling rate. The cooling time had a negligible effect on the gloss in comparison to the effect of filling rate. Ageing at 100 °C for 240 h increased the gloss level, especially at the far end of the moulding, but only for specimens manufactured at the lowest filling rate. Roughness measurements with a profilometer showed that the gloss development during processing and ageing was related to the topographical matching between the developing polymer surface and the mould topography.